208 The American Geologist. o.-tober L905 
gated into successive life zones similar fb those which have 
long been developed in invertebrate, palaeontology; these 
life zones in some cases subdivide not only the periods 
(Eocene, Miocene, etc.), but also subdivide the stages 
i Bridger, Unita), etc. Second, not only have these clearer 
chronological subdivisions been made, hut the faunas have 
been separated according to their kinds and the nature of 
the deposits, into those which inhabited respectively the 
lowlands and rivers, forests, plains, and uplands. The ad- 
vance of physiography has been felt, and by the careful 
work of Hatcher 1 , Matthew", and Gidley 8 , the theory of 
fluviatile. flood plain, and aeolian deposits has tended to re- 
place the theory of great lakes or lacustrine deposit-. Third, 
there has accordingly been brought about a modification of 
our views as to the meteorological or climatic phases of the 
Tertiary period, in the direction of extending- the idea of the 
existence of great dry plains with drifting sands favorable 
to Aeolian deposits chiefly in the Lower Pleistocene, Plio- 
cene and .Miocene ; we speak less of a moist, subtropical, and 
more of a drier climate. Fourth, the zoogeographical rela- 
tions of the North American faunas to those of other con- 
tinents have become much more clearly understood (Os- 
born 6 ) in connection with more exact geological records not 
only by the addition of many new forms from the Eurasiatic 
radiation hitherto unknown, but also by observing more 
precisely the time of arrival of Eurasiatic migrants in the 
Lower, Mid- and Upper Miocene and of South American 
in the Pleiocene. Fifth, the phylogenetic succession has 
become much clearer and more direct, although a vast 
amount remains to be done. The separate branches of the 
mammalian phyletic tree have been successfully traced back 
farther and farther toward the beginnings of the Tertiary, 
1 Origin of the OUgocene and Miocene Deposits of the Great Plains 
Proc. Amer. Philos. Soc, xli. No. 169, Apr. 1902. 
2 Is the White River Tertia/v an Aeolian Formation Amer. Nat., 
xxxiii. May, 1899, pp. 403-40S. 
3 Fossil Mammals of the Tertiary of Northeastern Colorado, Mem. 
Amer. Mus. Xat. Hist., vol. 1. pt. vii, Nov. 1901. 
■i The Fresh-water Tertiary of Northwestern Texas. Bull. Amer. Mus. 
Xat. Hist., vol. xix, 1903, pp. 617-635. 
5 Matthew and Gimlet. New or Little Known Mammals from the 
Miocene of South Dakota. Bull. Amer. Mus. Xat. Hist., vol. xx, \>\> 241- 
Faunal Relations of Enrove and America. Science, vol. xi, \]nl. 
pp. 161-514. 
